1) While Ramis worked with them on Year One, it was Sony that hired Lee Eisenberg and Gene Stupnitsky to scribble a GB3 script – without consulting Reitman, Ramis, or Aykroyd. In fact, Sony has been pushing HARD for this movie – moreso than the principle actors/directors/writers involved.

2) Reitman is laughing off Murray as a ghost, but doesn’t deny it. In fact, he makes it sound like Murray is actually still talking about being a ghost, but since the script and deal isn’t a lock, nobody is committing to the idea.

“We’re well aware of his interest,” Reitman said with a chuckle when asked whether Murray’s Venkman will be going ectoplasmic. “I’ve had some wonderful conversations with him—and that’s all I’ll say.

To put it another way, Murray is talking about being a ghost – publicly to the press and, as Reitman suggests, privately to him – but that’s all it is at the moment. Talk. This also accounts for Weaver’s comments – it’s more than reasonable that the possibility came up in conversation, which she told the press, which made it sound confirmed, when really it hasn’t been agreed upon yet.
3) Everyone is, as we have surmised, interested, but there continues to be no deal and no start date, so there is, as yet, no official lock on the 2011 release. It can still happen, but there are a lot of hurtles that must be jumped first.